Oak wilt

Oak wilt is a lethal fungal disease caused by the pathogen Bretziella fagacearum. This fungus colonizes the xylem of the tree, which is responsible for transporting water and nutrients from the roots to the crown. Once the fungus enters these vessels, the tree responds by producing tyloses and gums to plug the vessels, eventually causing the tree to starve and die from lack of water.

The disease primarily affects various species of oak trees. Members of the red oak group are highly susceptible and often experience rapid wilting and mortality. White oaks are generally more resistant and may survive for several years, though they eventually succumb to the infection or decline significantly in health.

Early symptoms include bronzing or browning of leaves, which typically starts at the margins and progresses toward the base. Affected trees often shed their leaves while they are still partially green. Internal diagnostic markers include dark brown or black streaks or discoloration in the sapwood, which can be seen upon removing the bark or cross-sectioning branches.

The fungus spreads via two main pathways: local transmission through root grafts between adjacent oak trees, and long-distance spread facilitated by sap-feeding beetles. These beetles visit fungal mats—structures that grow beneath the bark of infected trees—and then transport spores to healthy trees through open wounds caused by pruning, storms, or other mechanical damage.

Management of oak wilt focuses on prevention, as there is no cure for an infected tree. Key practices include creating root-graft barriers through trenching to prevent underground spread, strictly avoiding pruning of oaks during the spring and early summer when insect vectors are most active, and the rapid identification and removal of diseased trees to reduce local inoculum sources.